There was some discussion last year, and Noel was involved, about various possible implementations of a scratch-pad-like environment, but I haven't heard anything further about it since. It's already possible, with a bit of creative lateral thinking, to move 'blocks' around to try arrangement ideas, and while it's not non-destructive as the Studio One approach or as comprehensive as the Reason Blocks approach, it's pretty easy to fork and save a new version of the project and try things out without losing the original. I do this a lot.
The Matrix View also already offers a lot of arrangement possibilities if you explore it a little - I've used it to 'perform' an on-the-fly arrangement of a set of loops, sections and such, and while it could use some further evolution to reach the level of, say, Ableton in this regard, it's quite usable and seems to be still actively maintained, i.e. not abandonware within the product (it got some love in the most recent update in fact).
Don't get me wrong: I'm completely in favor of further development along these lines to enhance Sonar's capabilities in this area, but a lot can be done with the platform right now, and I've personally found that moving from a 'give me explicit features that allow me to do exactly what I want' mindset to a more 'I wonder how I can use what I already have to do something along these lines that might actually turn out to be interesting and cool' one has been liberating and creatively fruitful...
Here's a couple of threads with relevant discussion:
http://forum.cakewalk.com/Looking-for-a-way-to-duplicate-Studio-Ones-quotArranger-Trackquot-functionality-in-Sonar-m3295868.aspx http://forum.cakewalk.com/An-arrange-window-m3104937.aspx